When using an NDT/NDI device, such as an eddy current flaw detector inspecting a test object with a large thickness, low or very low operating frequencies are required. Some instruments are designed to operate at frequencies as low as 10 Hz. The frequency at which the instrument operates is referred to as the “operating frequency”. Under most testing scenarios, the instrument sends a strong excitation signal to the probe to form the eddy current in the material under test. The instrument is tasked to measure very small changes to the phase and amplitude of the signal returned to the probe as it is moved across the surface of the test object. The signal returned from the probe is amplified and sent to the detectors. Two synchronous detectors are normally used in existing practice to determine the phase angle of the received signal. The detector channel that detects the “in phase” component is referred to as the Real detector and the detector that detects the 90 degree shifted component is referred to as the Imaginary detector. Most existing methods of measuring the amplitude of an AC signal produce a DC signal with a very large ripple. Most of this ripple has a frequency twice the operating frequency and a smaller amount has the same frequency as the operating frequency. The real and imaginary detector outputs are then sent to a pair of identical filters. The filtered outputs detectors are then used for the measurement and for display.
The most common way of displaying the signals is to display a moving dot on an impedance plane displayed on a display screen. The output from the real detector and filter typically moves the dot up with increasing amplitude; the output from the imaginary detector and filter typically moves the dot to the right if the signal lags the excitation signal or to the left if the signal leads the excitation signal.
The difficulty with this method is with the filters used in existing technologies. In prior designs such as that was disclosed in U.S. Pat. No. 7,362,096, a low pass filter or a pair of low pass filters are used. Low pass filters are known to have a “roll-off” or “cut-off” frequency, the frequency above which signals are filtered out. Typical low pass filters require the unwanted signal to have a frequency several times the roll off frequency of the filter. When the low pass filter is set to a very low frequency, the instrument responds to signal changes very slowly. When the low-pass filter roll off frequency is increased to increase the instrument response, some of the AC signal leaks through the low pass filter. The leaked signals from the two low pass filters are 90 degrees from each other in phase and therefore they collectively move the dot in a circle. The problem is often referred to as “circles” or “circling dot” and a solution to it is long sought in the field of eddy current testing. The circular motion around the dot on the display where it should be stationarily located makes it very difficult to interpret the results. As a result, it hinders the inspection productivity.
The same problem and limitations are present in both analog and digital circuit designs. The same problem also exists for testing instrument of other technologies such as ultrasonic and Hall Effect testing when undesirably leaked signals of two orthogonal phases present.
Accordingly, a solution is needed to overcome the drawbacks described above and to achieve advantages of higher readability and accuracy of testing results and improved inspection productivity.